Oxygen Storage (OS) materials are well known solid electrolytes based on, for example, Ceria-Zirconia (CeO2—ZrO2) solid solutions. They are a ubiquitous component of aftertreatment catalysts for gasoline vehicles due to their ability to ‘buffer’ the active components in the catalyst against local fuel rich (reducing) or fuel lean (oxidising) conditions. OS materials do this by releasing active oxygen from their 3-D structure in a rapid and reproducible manner under oxygen-depleted transients, regenerating this ‘lost’ oxygen by adsorption from the gaseous phase when oxygen rich conditions arise. This reduction-oxidation (hereafter redox) chemistry is attributed to the Ce4+⇄Ce3+ redox couple, with the oxidation state of Ce depending upon local O2 content. This high availability of oxygen is critical for the promotion of generic oxidation/reduction chemistries e.g. CO/NO chemistry for the gasoline three-way catalyst, or more recently for the direct catalytic oxidation of particulate matter (soot) in the catalysed diesel particulate filter (CDPF) e.g. US2005 0282698 A1.
Hence there have been extensive studies on the chemistry, synthesis, modification and optimisation of Ce—Zr based OS materials. For example, the use of Ceria-Zirconia materials doped with lower valent ions for emission control applications have been extensively studied e.g. U.S. Pat. No. 6,468,941, U.S. Pat. No. 6,585,944 and US2005 0282698 A1. These studies demonstrate that lower valent dopant ions such as Rare Earth metals e.g. Y, La, Nd, Pr, etc., Transition metals e.g. Fe, Co, Cu etc. or Alkaline Earth metals e.g. Sr, Ca and Mg can all have a beneficial impact upon oxygen ion conductivity. This is proposed to arise from the formation of oxygen vacancies within the cubic lattice of the solid solution which lowers the energy barrier to oxygen ion transport from the crystal bulk to the surface thereby enhancing the ability of the solid solution to buffer the air fuel transients occurring in the exhaust stream of a typical gasoline (three-way) catalyst application.
Additionally it has been shown (U.S. Pat. No. 6,468,941 and U.S. Pat. No. 6,585,944) that the use of specific examples of the above dopants can provide full stabilisation of the preferred Cubic Fluorite lattice structure for Ceria-Zirconia solid solutions, with Y being identified as having particular benefit. The presence of the preferred Cubic Fluorite structure has been found to correlate with the most facile redox chemistry for Ce4+⇄Ce3+, from both the surface and bulk of the crystal, thus dramatically increasing the oxygen storage and release capacity, as compared to bulk CeO2. This benefit is especially pronounced as the material undergoes crystal growth/sintering due to the hydrothermal extremes present in typical exhaust environments. The incorporation of especially Y and to a lesser extent La and Pr have also been demonstrated to limit or, in certain cases, circumvent the disproportionation of the single cubic phase Ceria-Zirconia into a composite consisting of more Ce-rich cubic phases and more Zr-rich tetragonal phases, a process which results in marked decrease in redox function, surface area etc. of the solid solution.
Finally U.S. Pat. No. 6,468,941 and U.S. Pat. No. 6,585,944 teach the potential for employing base i.e. non-precious group (Pt, Pd, Rh, Au etc.) dopant metals into the Cubic Fluorite lattice of the solid solution as an alternative means to promote the redox chemistry of Ce, with Fe, Ni, Co, Cu, Ag, Mn, Bi and mixtures of these elements being identified as of particular interest. Hence while non-promoted OS materials typically exhibit a redox maximum, as determined by H2 Temperature Programmed Reduction (TPR), at ca. 600° C., the inclusion of base metals within the lattice can decrease this temperature by >200° C. or more at a fraction of the cost incurred by the use of precious metals.
However, while these base metals can be beneficially incorporated in the CeZrOx lattice and this incorporation can significantly promote low temperature redox function for fresh materials, the addition of these elements can also decrease fresh and aged phase purity and significantly decrease hydrothermal durability (promote crystal sintering and material densification), leading to losses in aged performance cf. base compositions without additional base metal. In addition during conventional aging cycles reactions may occur between the gas phase and the CeZr material which can result in extraction of these additional base elements from the Cubic Fluorite lattice. This in turn can result in formation of separate bulk phase(s) with low intrinsic catalytic activity or in a worst case scenario, phases which directly interact with the OS or other catalyst component resulting in a direct or indirect poisoning of the catalyst.
Thus, the aforementioned materials are potentially limited in their scope. For example, while lower valent ions may be successfully incorporated in the synthesis of a solid solution this can only be achieved by careful control of the synthesis and within specific limits for the final composition. This is necessary to ensure both the electrical neutrality and the preservation of the favoured Cubic Fluorite single-phase structure of the resultant compound. Hence, for example, the synthesis of an OS material containing a specific low valent base metal promoter ‘doped’ into a Cubic Fluorite structure with high Ce (>50 mol %) and/or low Zr (<30 mol %) contents is not facile and there is significant potential that the synthesis could result in a material with disproportionation into Ce-rich and Ce-poor domains, with a marked decrease in performance.
Similarly great care must be taken to balance the ultimate electrical ‘charge’ of the solid solution, hence the incorporation of Nb5+ in the cubic lattice may also be achieved but only by introduction of equimolar quantities of Y3+, in order to preserve the overall cationic charge balance of 4+. Again any imbalance or heterogeneity of Nb/Y content within the local crystal structure is undesirable and could lead to phase stability and purity issues with ultimate loss of required redox function as outlined in U.S. Pat. No. 6,605,264.
A further, and perhaps more significant, drawback of introducing low valent base metal ions within the Cubic Fluorite lattice is that the ions are dispersed throughout the bulk of the crystal structure and thus the surface concentration of the ions may be very low. This in turn limits the extent of the dopant ions to interact directly with the exhaust environment. Thus while it is possible to dope to Sr, Ca and Mg etc. into the cubic lattice the ability of these ions to provide additional chemical functionality e.g. as a NOx trap to provide transient adsorption of NO and NO2 is limited by the available concentrations of ions in the surface and immediate sub-surface of the crystal.
What is needed in the art are stable OIC/OS materials with facile and high oxygen storage and oxygen ion conductivity properties for any practical Ce-content. Moreover, greater flexibility of composition is required without the current penalty of decreased durability and activity. Finally the manner by which these enhancements are realised should be facile and robust with minimal process steps for maximum throughput.